Technical Field
The following disclosure relates to an image forming apparatus having a beam scanning device configured to scan and expose a photoconductor using a polygon mirror and a method of controlling the beam scanning device.
Description of Related Art
There is known an image forming apparatus equipped with a beam scanning device including: first and second light beam sources; a polygon mirror configured to reflect first and second beams respectively emitted from the first and second light beam sources; a first scanning optical system disposed on one side of the polygon mirror for focusing the first beam on a first photoconductor; and a second scanning optical system disposed on another side of the polygon mirror for focusing the second beam on a second photoconductor. In the known apparatus, an optical sensor is provided upstream of the first scanning optical system in a scanning direction of the first beam for detecting the first beam, whereas no optical sensors are provided for the second scanning optical system.
In the known apparatus, scanning exposure of the first beam is started when a predetermined first-beam exposure standby time elapses after detection of the first beam by the one optical sensor, and scanning exposure of the second beam is started when a predetermined second-beam exposure standby time elapses after the detection of the first beam. (Here, the first-beam exposure standby time is defined as a time period from a timing of detection of the first beam to a timing of start of the scanning exposure of the first beam, and the second-beam exposure standby time is defined as a time period from the timing of detection of the first beam to a timing of start of the scanning exposure of the second beam.) Further, the known apparatus determines the second-beam exposure standby time based on time intervals at which the one optical sensor detects the first beam, in consideration of an error in surface division accuracy of the polygon mirror, namely, in consideration of an error in the angle among different mirror surfaces of the polygon mirror. Thus, a position of an image formed by the first beam and a position of an image formed by the second beam are aligned in a width direction of a sheet.